It is well-known in the practice of spectral sensitization of silver halide emulsions for color photographic use that cyanine dyes used for this purpose typically J-aggregate upon adsorption to the silver halide crystal. (This is not to assert that J-aggregation is typical of cyanine dyes--only that it is a characteristic property of such cyanine dyes as are useful for photographic purposes.) A discussion of J-aggregation can be found in T. H. James, editor, The Theory of the Photographic Process, 4th Edition, Macmillan, N.Y., 1977. The discovery of two cyanine dyes which will, when applied to the emulsion simultaneously, form a so-called "mixed aggregate" has been a topic of great interest in the study of photographic science. See, for example, Y.Yonezawa, T. Miyama, and H Ishizawa, J. Imaging Sci. Technol., 39 331(1995); V. Bliznyuk and H. Mohwald, Thin Solid Films, 261 275 (1995); T. L. Penner and D. Mobius, Thin Solid Films, 132 185 (1985) and G. Scheibe, A. Mareis, H. Ecker, Naturwiss, 29 474(1937).
The phenomenon has much to offer in the practice of photographic science. For example, the practice of spectral sensitization would no longer be constrained by the position of single dyes; rather, mixtures of dyes could be used to manipulate the location of spectral sensitization with impunity. This would provide great value to photography, as often the light output of the image or scene to be photographed is not in harmony with the light-capturing location of the presently available sensitizing dyes. However, the literature reports only isolated examples of dyes which have been found to form a mixed aggregate, and the physical rules which govern this behavior are only qualitatively known, as is evidenced by the following statement from Bliznyuk and Mohwald: "However, little is known about the molecular properties that determine miscibility or immiscibility. This is unfortunate, because mixed aggregates are very promising for various reasons." The extent of the qualitative understanding is simply that dyes must be sufficiently similar sterically to be compatible in the mixed aggregate, and that their individual aggregates may not be too distant from one another energetically. For example, Yonezawa et al. state that "it is plausible" that "nearly equal" positions of the two individual aggregate positions "favor the formation of the HA aggregate" (HA is defined as "homogeneous aggregate").